Surface evolution of strained SrRuO 3 films deposited at various temperatures on SrTiO 3 (001) substrates

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Chan Gyung Park Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea (Received 3 December 2005; accepted 16 March 2006)

Surface evolution was studied for strained SrRuO3 films with a nominal 75 nm thickness deposited at various substrate temperatures (650–850 °C). Epitaxial growth of the films was achieved on single TiO2-terminated SrTiO3 (001) substrates by using ion-beam sputtering. The surface morphology of the deposited films was investigated by scanning tunneling microscopy in ambient conditions, and their microstructure was characterized by transmission electron microscopy. The self-organized step-terrace structure was observed for the films deposited at lower than 800 °C, suggesting that the epitaxial growth proceeded by step-flow growth. In particular, each film showed characteristic surface evolutions pertinent to the misfit strain relaxation stage, mostly influenced by the moving segment of misfit dislocations threading up to the surface: surface undulations for the film at the initial stage of relaxation (deposited at 650 °C), circular growth spirals during the relaxation stage (700 °C), and well-ordered step-terrace structure after almost full development of misfit dislocations (750 and 800 °C).

I. INTRODUCTION

SrRuO3 is a metallic conductive oxide that crystallizes in orthorhombic distorted perovskite-type structure with the lattice parameters of a ⳱ 0.5538 nm, b ⳱ 0.5573 nm, and c ⳱ 0.7856 nm at room temperature.1 Because of the small distortions in RuO6 octahedra arrangement, the crystal can be regarded as a pseudo-cubic structure with a lattice parameter of a⬘ ⳱ 0.393 nm. SrTiO3 has a lattice parameter of 0.391 nm and is a standard substrate used for the epitaxial growth of perovskite films. The SrRuO3/ SrTiO3 (001) system shows the smallest known lattice misfit (0.64%) in perovskite oxide combinations. The heteroepitaxial films have been extensively explored recently as bottom electrodes or buffer layers for growing high-quality ferroelectric and superconducting oxides because the grown SrRuO3 films are electrically conductive, structurally compatible, and chemically stable when in contact with most of the perovskite oxides.2–4

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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0187 1550

http://journals.cambridge.org

J. Mater. Res., Vol. 21, No. 6, Jun 2006 Downloaded: 14 Jul 2014

The step-flow growth of individual layers is most desirable for the successful integration of ultrathin oxide films for a noble heterostructure device. In principle, step-flow growth on a vicinal substrate can be achieved when the surface diffusion length of adatoms is larger than the surface terrace width to incorporate in the kink sites at terrace ledges. Control of nucleation in this manner can reduce surface inhomogenities as well as crystallographic defect density. Misfit dislocations appearing at heteroepitaxial interfaces are equilibrium defects that can relieve the unstable buildup of misfit strain over a

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